Semiconductor lasers and amplifiers are valued for their small size, low cost, robustness, high efficiency and a wide range of available wavelengths. However, it has proved to be very difficult to obtain both high power and good spatial coherence from such devices. Single-stripe devices are known to produce high quality output beams, but are generally limited to at most a few hundred milliwatts of output power, at which point any further increase of the optical intensity at the output facet begins to affect device reliability. Broad-area lasers and amplifiers show great promise for reliably producing higher output powers, but suffer from modal instabilities, collectively known as optical filamentation, that seriously degrade the spatial coherence of their output beams. For example, simple Fabry-Perot broad-area lasers do not exhibit diffraction-limited far field outputs at all under cw operation. And while broad-area amplifiers injected with a uniform input beam have been demonstrated that provide quite high diffraction-limited optical power (to date, up to 3.3 W cw and 11.6 W pulsed from diode laser injection and 21 W pulsed from Ti:Sapphire solid-state laser injection), any further improvement in their performance is ultimately limited by the onset of filamentation. It is not surprising then that understanding the causes of filamentation and finding a solution to this problem, in order to attain higher power spatially coherent outputs from broad-area lasers and amplifiers, has been a goal of researchers for many years.
The phenomenon of filamentation has been studied extensively and a number of detailed analyses have been done on the formation and growth of optical filaments in broad-area semiconductor diode lasers and amplifiers. Published studies include A. H. Paxton and G. C. Dente, "Filament formation in semiconductor laser gain regions", Journal of Applied Physics, vol. 70, no. 6, 15 Sep. 1991, pages 2921-2925; Robert J. Lang, et al., "Spatial evolution of filaments in broad area diode laser amplifiers", Applied Physics Letters, vol. 62, no. 11, 15 Mar. 1993, pages 1209-1211; and Robert J. Lang et al., "Spontaneous Filamentation in Broad-Area Diode Laser Amplifiers", IEEE Journal of Quantum Electronics, vol. 30, no. 3, March 1994, pages 685-694. The analyses show that filamentation is a result of an optical nonlinearity characteristic of semiconductor light amplifying media that causes a runaway effect known as self-focusing. In such media, the index of refraction (n) is a function of the optical power (P) propagating through the medium. In particular, the media exhibit a positive change in the refractive index with optical intensity (i.e., .differential.n/.differential.P&gt;0). Because of this characteristic, any lateral perturbation in the optical field propagating in the medium, in the form of a local increase in the optical intensity, causes a local increase in the index of refraction that tends to focus and further magnify the perturbation. Such self-focused localized regions of increased intensity, commonly called filaments, tend to grow exponentially in intensity as the beam propagates through the medium, and the greater the self-focusing parameter, defined as the ratio of change of index of refraction per change in incident optical power, .differential.n/.differential.P, the more rapid the growth.
The physical mechanism in semiconductor amplifying media which results in this self-focusing effect and the consequent optical filamentation is gain saturation and the spatial hole burning of carriers in combination with a relatively strong dependence of the index of refraction on gain and carrier density. While there is also a thermal component of filamentation due to the relation between local temperature and the refractive index, the primary effect appears to be carrier-based. Semiconductor amplifying media exhibit strong coupling between the gain and refractive index in the medium. This gain-index coupling can be represented by a ratio (.alpha.), variously known as the antiguiding factor, the linewidth enhancement factor, or the .alpha.-parameter. The antiguiding factor (.alpha.) is defined here as the ratio between the change in index of refraction (n) per change in carrier density (N) and the change in gain (.gamma.) per change in carrier density (N), scaled and normalized to be dimensionless, i.e., .alpha.=(4 .pi./.lambda.)(.differential.n/.differential.N)/(.differential..gamma./.di fferential.N), where .lambda. is the optical wavelength. The antiguiding factor (.alpha.), as defined here, is generally negative in semiconductor amplifying media. Any increase in gain due to the injection of carriers is accompanied by a significant decrease in refractive index, producing antiguiding of light propagating in the medium. Conversely, any local increase in the optical intensity in the medium depletes the carriers, causing a local decrease in the carrier density and a corresponding local increase in the effective refractive index, which results in self-focusing and a further increase in local intensity. Theoretical models show that the rate at which self-focusing occurs scales with the magnitude of the antiguiding factor. (Readers should note that the published literature provides a variety of definitions for the antiguiding or linewidth enhancement factor .alpha., some identical to that given above and some differing from the above definition by a constant or wavelength-dependent proportionality factor. Some definitions include a negative sign so that .alpha. will be positive, while others do not.)
As previously noted, broad-area lasers and amplifiers are particularly susceptible to the adverse effects of self-focusing, namely filamentation. In lasers, the regenerative nature of laser oscillation can explain the spontaneous generation of filaments in the near field. In amplifiers subject to external injection, tiny random spatial fluctuations in the optical input grow into filaments as the beam propagates down the amplifier. Such fluctuations are inevitable due to imperfect optics. Small inhomogeneities in the media itself or in the pumping of the media may also contribute to the generation of the intensity perturbations that lead to filaments. Despite the random nature of the input fluctuations, the observed filaments often have a remarkably regular pattern, especially in double-pass amplifiers, because the rate at which periodic perturbations grow varies with the period of the perturbation and depends upon wavelength, saturation, the differential gain and the antiguiding factor in the amplifier. The period of perturbations that experiences the highest rate of growth is called the peak filament period and is typically 5-20 .mu.m in broad-area semiconductor amplifying regions. Because the gain is more completely saturated in double-pass amplifiers, the perturbations generally become so unstable that the optical field collapses into a periodic array of filaments. In any case, the filaments impress both intensity and phase variations upon the optical field, which destroy the spatial coherence of the output beam.
In addition to the loss of beam quality due to filamentation, the gain-index coupling characterized by the .alpha.-parameter has other undesirable effects that affect even single-stripe devices. These include a substantial linewidth broadening that degrades the spectral purity of the output beam, and a modulation-induced chirp that limits the useful modulation speed of laser diodes well below their fundamental limit. Much of the research regarding the .alpha.-parameter has been directed toward its effect on devices for optical communication. Linewidth broadening in semiconductor lasers and amplifiers is analyzed by K. Valhala et al. in "On the linewidth enhancement factor .alpha. in semiconductor injection lasers", Applied Physics Letters, vol. 42, no. 8, 15 Apr. 1983, pages 631-633, and Kerry Hinton in "Optical Carrier Linewidth Broadening in a Traveling Wave Semiconductor Laser Amplifier", IEEE Journal of Quantum Electronics, vol. 26, no. 7, July 1990, pages 1176-1182. Valhala et al. note that linewidth broadening is proportional to (1+.alpha..sup.2).
Reductions in the measured value of .alpha. have been reported for multiple quantum well (MQW) lasers by C. A. Green, in "Linewidth enhancement factor in InGaAsP/InP multiple quantum well lasers", Applied Physics Letters, vol. 50, no. 20, 18 May 1987, pages 1409-1410, strained MQW lasers by N. K. Dutta et al. in "Linewidth enhancement factor in strained quantum well lasers", Applied Physics Letters, vol. 56, no. 23, 4 Jun. 1990, pages 2293-2294, and strained and p-doped MQW lasers by A. Schonfelder et al. in "Differential Gain, Refractive Index, and Linewidth Enhancement Factor in High-Speed GaAs-Based MQW Lasers: Influence of Strain and p-Doping", IEEE Photonics Technology Letters, vol. 6, no. 8, August 1994, pages 891-893.
In U.S. Pat. No. 5,208,822, Haus et al. disclose a semiconductor laser diode in which the net optical gain or refractive index is kept almost constant under modulation in order to realize a device with reduced chirp. The laser structure has an active region comprising two regions optically coupled to each other with the second region having a higher bandgap energy than the first region. The oscillation wavelength is selected by a feedback grating such that the two regions have .alpha.-parameters with opposite signs. Separate electrodes for each region provide independent current injection to the two regions. For amplitude modulation, the changes in gain for the two regions have the same sign, so that the changes in refractive index have opposite sign to keep the phase condition in the cavity unchanged for ultra-low chirp. For frequency modulation, net optical gain is kept almost constant by opposite changes in current injection for the two regions, so that the respective changes in refractive index have the same sign, thereby changing the lasing wavelength. Haus et al. disclose a narrow stripe active region and make no mention of either self-focusing or filamentation in broad-area devices.
An object of the invention is to reduce or eliminate the self-focusing and resultant filamentation in broad area semiconductor lasers and amplifiers, thereby providing higher power coherent light output from such devices.